The importance of small airways has been recognized in recent years in terms of the pathogenesis and early detection of obstructive lung disease and in terms of their role in controlling the distribution of ventilation. A quantitative investigation of the basic mechanical and pharmacological properties of the excised rabbit bronchiole (250 to 1000 micrograms) will be conducted. The experiments will be done using cylindrical segments mounted in a specially constructed myograph. The method has the advantage of minimally disrupting the in vivo architectural arrangement of the smooth muscle cells in the airway wall. In this way a more valid physiological interpretation of the results of the measurements of the active and passive circumferential length-tension relationship and the active and passive stiffnesses of the bronchiolar wall can be made. The effects of bronchiolar properties of alterations of the pH, PCO2 and PO2 of the bathing medium, as well as several pharmacological agents will be evaluated. Light and electron microscopy of the bronchiole will be conducted in order to correlate the measured mechanical properties with the anatomical arrangement of the smooth muscle cells and other constituents in the bronchiolar wall. These experiments will provide fundamental data on small bronchioles which is not currently available. It will also afford a model for the future evaluation of therapeutic agents and the effect of various disease states on peripheral airway properties.